The present invention relates generally to mixing apparatus and more particularly to an apparatus for mixing liquids with liquids, liquids with solids and liquids with gases contained in a vessel. As used in this application, the term "fluid" includes, but is not limited to, all of the above.
Mixing of liquids or liquid suspensions in a vessel generally requires an impeller on the end of a shaft being driven to create flow fields in the fluid in the vessel. The mixing apparatus is designed to achieve a desired degree of mixing of the liquids, solids or gases in the liquid. Depending upon the depth of the tank, the viscosity of the liquids and the type of impeller, one or more impellers may have to be used. The use of multiple impellers on a single shaft is well known in the prior art to accomodate vessels of increased depth and high viscosity material. In order to produce a single flow field, pitch blade turbine must be closely spaced, for example, within 1/2 to 3/4 the blade diameter apart. Otherwise, substantially independent plural flow fields and thus levels or vertical zones of mixing results as illustrated in FIG. 1. Since the rotational speed of the shaft is the same for all impellers on a single shaft, the power cannot be independently adjusted for each level of the liquid. This restricts the ability to perform certain chemical processes which require different power requirements at different levels at different times in the process.
The prior art has attempted to provide concentric drive shafts to allow individual speed control of coaxial impellers. The disadvantages of this system is that the gear drive and shaft for the upper-outer mixer are very expensive. Also, the bore through the reducer and shaft must be sufficiently large to allow the lower shaft to pass through and also leave room for shaft deflection.
Certain applications require different degrees of mixing at different periods. For example, in a solid suspension, the uniformity of suspension is desired during dispensing while a low degree of suspension short of complete settling is desired during periods between dispensing. Variable speed drivers have been used, but have not been satisfactory. Since power is a function of the cube of speed, it is difficult to adjust the speed to obtain the desired power. Prior art systems are generally designed for a single mode of operation, for example, uniform mixing or suspension. In a start-up situation where a lower mixer may be encased in sediment, various solutions have been attempted. One solution is to raise the lower mixer above the sediment before activating. This requires an extensive support and lifting system. Alternatively, others have maintained mixing which is an inefficient use of energy.
As used in this application, a "single flow pattern" is where for each vertical plane originating and extending radially from the center axis of a cluster of mixers, there is only one null point. A null point is where the mean velocity is zero and the flow in the vertical plane circulates about this point. The locus of null points formed by all such vertical planes is a single closed loop. The flow pattern can thus be described as a toroid about the single closed loop. Obviously, this excludes small secondary flow patterns around baffles, corners, and other localized disruptions in the mixing vessel. If there is total symmetry; if the secondary mixers rotate in a direction that is opposite the direction of rotation of the first mixer; if the mixing vessel is cylindrical; and if no baffles are used, then the closed loop will be a circle.
Thus, it is an object of the present invention to provide a multiple impeller system which generates a single flow pattern and affords maximum operating flexibility.
Another object of the present invention is to provide an impeller system design for relatively deep tanks to provide substantially axial flow through mixers thus developing an efficient top to bottom mixing pattern.
A further object of the present invention is to provide a mixing apparatus which is energy efficient by being able to maintain flow without maintaining homogeneity in the fluid.
Still a further object of the present invention is to provide a mixing system having the ability to efficiently and economically produce different degrees of mixing at different times in an operating cycle.
An even further object of the present invention is to provide a mixing system capable of generating a single flow pattern even with some impeller failures.
A still even further object of the present invention is to provide a multi-level impeller system which is energy efficient in all modes of operation.
Even a further object of the present invention is to provide a mixing system which eliminates the need for vertical baffles.
These and other objects of the invention are attained by providing at least two levels of mixing, one adajcent the top of the vessel and one adjacent the bottom of the vessel and individually driven at appropriate speeds to produce a single flow pattern in the liquid in the vessel. The pumping of each level is designed for the desired degree of mixing at that level while maintaining the single flow pattern. The bottom mixing apparatus is preferably a single large diameter impeller whereas the top mixing apparatus includes a plurality of smaller diameter impellers. The plurality of upper level mixers are positioned equally distant from and symmetrical about the lower impeller's vertical axis of rotation. Each of the impellers have a substantial axial or converging flow field exiting the impeller. For vessels of even greater depth and for other systems or processes requiring more levels of mixing, an additional plurality of impellers may be provided between the top plurality and the bottom single impeller driven so as to produce a single flow field with appropriate pumping at each level of mixing. As an alternative, the plurality of impellers may be substantially spaced from the single impeller and operated to produce pumped flow in the opposite direction of the single impeller; producing the single flow pattern.
Vertical baffles can be eliminated by chosing the impellers that rotate in opposite directions while developing a single flow pattern in the vessel.
A controller activates and deactivates the individual impellers to achieve the desired degrees of mixing at different times or stages of a process. For impeller failures, the controller may deactivate other impellers to maintain the single flow pattern. The side and bottom of the tank meet at the interior of the tank at an angle greater than 90.degree. C. to improve flow efficiency.
The use of a plurality of vertical rotational axis mixers with plural impellers on each drive shaft is well known in the prior art. These are generally interleaved so as to provide high shear force to blend very viscous materials. These are high shear systems with independent interweaving flow patterns not designed for efficient pumping. Similarly, the use of a large slow moving mixer to produce the gross flow of material and a smaller diameter, high speed mixer to produce high shear forces and no pumping adjacent thereto is also known in the prior art. This configuration is considered a single level of mixing. Generally high shear, small diameter mixers produces amalgamation or breaking action at the edge of the flow control mixer to introduce granular material into the overall mixture. Again, this is generally for fluids of high viscosity. The prior art also includes a plurality of horizontally spaced mixers each having separate and independent mixing zones. Although multiple mixers at various depths and locations in a container are known for many purposes, the concept of using plural mixers at different levels or locations to produce a single flow pattern is not shown by the prior art.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.